Provide "light" IDE theme option

[samajammin]

Summary

Provide a toggle option in UI for users to view the IDE with a "light" theme.

Motivation

Related to #248, a couple developers gave feedback in our recent survey that the dark theme is

• intimidating for new developers
• poor readability of READMEs w/ black & white scheme

Describe alternatives you've considered

• Do nothing
• Make smaller changes to the existing dark theme (e.g. Update code style on READMES #248 that improve readability)

Additional context

I doubt a theme rehaul will be a top priority but want to record this feedback. This will likely depend on effort required, e.g. if Monaco has an easy drop-in theme option, this could be a nice win.